This invention relates generally to anesthesia machines used to administer an anesthetic agent to a patient for anesthetizing the same.
Typically, such machines combine valving, flowmeters and needle valves to deliver nitrous oxide and/or oxygen to the patient and also have the facility for passing the gas (oxygen or oxygen-nitrous oxide) through an anesthetic vaporizer of the calibrated type, such that the gas picks up anesthetic vapor of a predetermined precise proportion. The gas thus contains a volatile and potent anesthetic agent for carrying the patient down to surgical anesthesia levels.
Such anesthesia machines normally allow a selection of the volatile, liquid anesthetic and, thus, provide suitable mounting means and the necessary valving to allow an operator to choose the anesthetic agent he intends to use. Common among such anesthetic agents are halothane and enflurane, and it is not unusual to have a vaporizer available for ready use for administration of halothane and a vaporizer for enflurane.
One difficulty in selector valves for choosing between a selection of anesthetic agents and the various flow paths and connections associated therewith, is that the liquid anesthetic agents, being volatile, continually emit vapor that may inadvertently seep into the flow of oxygen or oxygen-nitrous oxide when the selector valve is actually in the off position.
Similarly, when two anesthetic vaporizers are potentially in the circuit, it is a further problem that the operator has selected one anesthetic, yet vapors from the other anesthetic may seep into the flow of gas to the patient. In any case, the seepage of unknown quantities of an anesthetic agent to the patient is obviously undesirable.
In prior selector valves, there was, in some cases, the further potential problem that the operator might position, either by accident or by purpose, the selector valve in some intermediate position, that is, between a positive selection of a particular anesthetic vaporizer and the off position when no anesthetic vaporizer is in the circuit to the patient. In such case, again, the actual flow of anesthetic agent to the patient becomes unknown and, in view of the need to have a continuous, very accurate knowledge of the exact vapor concentration to the patient, the mid-positioning is also extremely undesirable.